Contact carrier for multiturn rotary potentiometer



Feb. 4, 1969 .HAL L 3,426,307

CONTACT CARRIER FOR MUL'IITURN ROTARY POTENTIOMETER Filed May 15, 1967 Sheet of S INVENTOR.

LLOYD E. HALL ATTORNEY Feb. 4, 1969 L. E. HALL' 3,426,307

LIONTACT CARRIER FOR MULTITURN ROTARY POTENTIOMETER Filed May 15, 1967 Sheet 2 of 5 FIG. 4

INVENTOR.

LLOYD E. HALL BY ATTORNEY Aw W 4 Filed May 15, 1967 Sheet 3 of Feb. 4, 1969- L. E. HALL 3,426,307

CONTACT CARRIER FOR MULTITURN ROTARY POTENTIOMETER 'INVENTOR.

LLOYD E. HALL ATTORNEY United States Patent 3,426,307 CONTACT 'CARRIER FOR MULTITURN ROTARY POTENTIOMETER Lloyd E. Hall, Fullerton, Calif., assignor to Beckman Instruments, Inc., a corporation of California Filed May 15, 1967, Ser. No. 638,457 US. Cl. 338143 4 Claims Int. Cl. H01c /00, 5/02 ABSTRACT OF THE DISCLOSURE A variable resistance device including a helically wound multiturn resistance coil mounted within a case and a rotor, driven by a shaft mounted within the case for rotatably positioning an electrical contact with respect to the resistance coil. The contact is carried on a carrier block which, in turn, traverses a support track on the rotor as the electrical contact moves along successive portions of the helically wound multiturn resistance coil. Support arms of the carrier block straddle the motor and ride against guide surface formed on opposite sides of the rotor to minimize movement of the contact carrier block about the support track during rotation of the rotor.

This invention relates generally to variable electrical resistance devices such as potentiometers, rheostats and the like and, more particularly, it is directed to an arrangement for reducing backlash in a rotary potentiometer.

Backlash in a potentiometer is usually defined as the maximum difference in contact position that occurs when the drive shaft is moved to the same actual output ratio point from opposite directions. When two members, such as the potentiometer rotor and the contact carrier block, are arranged for transmission of motion from one to the other, there is a certain amount of lost motion that results between the two members in reversing the direction of movement. This lost motion is caused by movement of one member without contemporaneous movement of the other due to spacial clearances between the members and the resistance to such motion by friction with outside or external bodies, such as the friction between the electric contact and the helical coil. This lost motion or backlash usually increases as the respective members Wear in their areas of mutual contact, resulting in greater looseness and inaccuracy. This is extremely disadvantageous when it occurs in a precision potentiometer in which the contact must be moved along a helical coil to reach a particular point of electrical potential with extreme precision during movement of the shaft from either direction of reference.

One means employed for overcoming backlash is shown in the Matheson et al. Patent 3,099,809, in which the carrier element for the contact is provided with guide arms designed to firmly clasp the shaft or rotor with such arms being continuously in compression against guideways formed on the rotor. While this arrangement may initially overcome the problem of backlash, it is extremely difficult to manufacture the mating parts so that the carrier slider block does not bind against the rotor and introduce other errors into the mechanism. The compression fit of the guide arms against the rotor track also causes undue wear of the carrier and track, ultimately resulting in a loose fit and the associated problems of backlash. The present invention is directed to an improved construction for an arch shaped contact carrier adapted to slide on a support track of a potentiometer rotor which construction does not require a compression fit between these members yet greatly reduces the backlash problems associated with looseness of fit.

3,426,307 Patented Feb. 4, 1969 Accordingly, it is an object of the present invention to provide an improved rotary potentiometer having a rotor member and contact carrier block member adapted to slide on a support track formed on the rotor and so designed as to permit considerable tolerance between the mutually engaging portions of these members yet minimizing backlash.

For a better understanding of the invention reference may be had to the accompanying drawings in which:

FIGURE 1 is an elevation view taken in cross-section of a multiturn potentiometer employing the rotor and contact block arrangement of the present invention;

FIGURE 2 is an end view of the rotor of FIGURE 1;

FIGURE 3 is an enlarged view taken from the top of a contact carrier block and illustrating the relative position of the electrical contact element mounted thereon;

FIGURE 4 is an enlarged assembly view of a rotor and contact carrier block mounted thereon in accordance with the present invention;

FIGURE 5 is another end view of a rotor and contact carrier block assembly employing the concepts of the present invention; and

FIGURE 6 is an end view of still another rotor and contact carrier assembly employing the concepts of the present invention.

Referring now to FIG. 1 there is shown a preferred embodiment of the present invention including a multiturn helically coiled resistance element 11 mounted within a cylindrically shaped housing 12. (The housing is preferably formed of a plastic or other moldable compound in which a helical groove 9 may preferably be molded during the formation of the housing.) As shown in FIG. 1, the helical resistance coil 11 is disposed within the helical groove 9 formed around the inner peripheral surface of the housing 12. The ends of the resistance element 11 are connected to terrinals 13 which provide conventional points for connecting the helical resistance coil into an external circuitry. A front lid 14 and a rear lid 15 are suitably attached to the housing 12 and provide support for a rotatable shaft 16 which is positioned substantially coaxial with respect to the axis of the helical coil 11.

Fixedly attached to the shaft 16 is a rotor 17 having at least one longitudinal support track or tracks 19 adapted to rotate in a plane substantially parallel to the surface of the helically wound coil 11. The rotor block 17 is preferably molded of an inexpensive non-conductive material, such as filled nylon, and the shaft 16 may be molded therein, as may be seen in FIG. 2. The rotor supports a contact carrier block 18 thereon which is adapted to move closely adjacent the helical coil and, during rotation thereof, to traverse the length of the rotor Sliding on the support tracks 19 of the rotor. The contact carrier block may also be formed of a non-conductive material such as filled nylon or the like.

In the embodiment of the invention illustrated in FIG. 2, there are a pair of support tracks 19 upon Which the contact carrier 18 may be positioned. These longitudinal support tracks are arranged at an angle with respect to the upper surface 8 of the rotor and are disposed in planes intersecting at approximately with respect to one another. The rotor 17 is also provided with a pair of fiat undercut surfaces 20 located on opposite sides of the shaft 16. Surfaces 20 are recessed a short distance thereby providing overhanging edges or shoulders 21 which give the rotor a cross-sectional appearance similar to that of a keyhole or arrow, with the point cut off; the shoulders 21 forming the barbs of the arrow shaped cross-section. Surfaces 20 are guide surfaces and are located a substantial distance below the support tracks 19 on the upper portion of the rotor.

As will be seen in FIG. 2, a groove 22. is provided on one side of the rotor block which is designed to hold a slip ring contact 23 (seen only in FIG. 1). The slip ring contact 23 makes continuous electrical contact with slip ring 34 mounted on end wall 15. Slip ring 34 is, of course, adapted to be connected int-o an external electrical circuit through means of a terminal (not shown). Electrically attached to the slip ring contact 23 is a flexible lead 24 which, in turn, is electrically connected to a contact element 26 supported on the contact carrier block 18. A circularly shaped end section 25 on the rotor acts as a thrust bearing and maintains the rotor 17 accurately positioned between end walls 14 and 15 on the shaft 16.

As may best be seen in FIGS. 3 and 4, the contact carrier 18 is generally arch shaped and is provided with two essentially parallel flanges 27 each of which are preferably adapted to ride within a helical slot 35 machined or otherwise formed in the housing 12 between the adjacent turns of the helically coiled resistance element 11. In may potentiometers the guide slot 35 is not provided and the flanges 27 may be guided merely by the coiled turns of the resistance wire. Referring to FIG. 3, it will be seen that the flanges 27 are preferably skewed to the helix angle of the helically wound coil for suitable alignment with the turns of the resistance element 11.

The wiper contact 26 is located between the flanges 27 and supported by the contact carrier for slidable engagement with the turns of the resistance element 11. The contact element 26 is appropriately mounted on the contact carrier, such as in a mounting hole 28, where the end of the contact element is electrically connected to the flexible lead 24.

In operation, the shaft 16 causes the rotor 17 to rotate. The contact carrier block 18 revolves with the rotor 17 and moves the wiper contact 26 progressively along the exposed inner surface of the helical resistance coil 11. The contact carrier flanges 27 guide within the helical groove 35 and cause the revolving contact carrier 18 to translate axially along the support tracks 19 of the rotor block 17. When the contact carrier 27 has traveled the length of the resistance element 11, a contact carrier edge 33 (see FIG. 4) abuts a stop (not shown) which protrudes from the rear lid 15. The contact carrier skirt or guide arm 30 then locks on the rotor block edge 21 and prevents further motion in that direction. By reversing the rotation of the shaft 16, the contact 26 proceeds back along the resistance element 11 in the opposite direction until the other contact carrier edge 33 abuts a stop (not shown) which protrudes from the front lid 14. The other contact carrier skirt or guide arm 30 then locks against the other rotor block edge and the contact 26 is prevented from moving any further in that direction.

As may be seen in FIG. 4, contact carrier 18 is positioned upon the rotor block in such a manner that the side skirts or guide arms straddle the rotor block 17. The carrier block is substantially free to slide on the support track or tracks 19 formed on the upper surface of the rotor block 17. The opening through the contact carrier block 18 is also formed substantially in the shape of a keyhole with the skirts or arms 30 adapted to be positioned over the shoulders 21 of the rotor. Arms or skirts 30 are provided with abutting surfaces 29 extending closely adjacent the guide surfaces 20 formed on the rotor block.

The upper support surfaces 31 of the arch shaped carrier block ride upon support tracks 19 formed on opposite sides of the carrier block. Surfaces 31 are preferably formed at an angle slightly greater than the angle of the surfaces or tracks 19 so that the contact carrier rides on the upper portion of the tracks 19. If the surfaces 19 are arranged at a 90 angle, it is preferable to have the surfaces 31 disposed at an angle with respect to each other of approximately 2 to 4 degrees greater, such as 92. In the embodiment illustrated in FIGS. 1-4, since the rotor block sides or tracks 19 have an angular relationship to one another of about this arrangement essentially allows mismatched angles between the contact carrier block surfaces 31 and support track 19. While the mismatch of the angles between the surfaces 31 and 19 of the contact carrier and the rotor respectively tends to cause a somewhat loose fit, the amount of rocking motion permitted by such loose fit is limited by the extremely close abutting relationship of the contact carrier skirt 30 and the guide wall 20 formed on the rotor. In practice it is very easy to make the space between the surfaces 29 of the skirt 30 to a very close tolerance only slightly greater than the width of the distance between the surfaces 20 of the rotor. This permits the respective surfaces to engage with a slip fit and minimizes the amount of rocking that may occur around the support track of the rotor.

Referring again to FIG. 4, note the dimensions D and D Dimension D represents substantially the distance between the point of engagement of the rotor block with the support track and the point on the contact which engages the coil. The contact member 26 is depressed to some extent, when it engages the coil, so that the distance D is somewhat less than that shown. The dimension D represents the distance between the support track 19 and the lower edge of the skirt 30 which engages the guide surfaces 20 of the rotor. These two distances D and D substantially represent the moment arms about the support track 19 about which forces act on contact carrier block 18 during rotation thereof. If the point 34 on the skirt 30 is colsely adjacent the guide surface 20 the amount of rocking about the points 19 of the support track is extremely minimal. By making the distance D; much greater than distance D movement of the point 34 about the pivot point of the support track 19 permits only a much smaller movement of the contact about the pivot of the support track. Thus, the greater the ratio of D /D the less the backlash for similar rotor blockcontact carrier fits. Thus, any ratio in which the moment arm D is equal to or greater than the moment arm D and the fit between the surfaces 20 and 29 (or at least between surfaces 20' and points 34) is close, the amount of backlash is reduced to a minimum.

Referring now to FIG. 5, there is shown another embodiment of the invention in which the support track of the rotor 51 is in the form of a sharp or knife edge 53. The carrier block 50 is provided with a groove 54 that is positioned directly on the knife edge 53 of the block 51. A pair of skirts 55 forming a part of the contact carrier block straddle the rotor 51 and closely abut the side surfaces 56 of the rotor block 51. As will be seen in FIG. 5 the ratio D /D of the moment arms is greater than 1. That is, moment arm D is substantially greater than moment arm D and this correspondingly reduces the amount of possible movement of the contact about the support track 53 of the rotor FIG. 6 shows still another embodiment of the invention. In this arrangement, the rotor is provided with a curved upper support surface. The V shaped groove 64 formed in the contact carrier block 60 permits the carrier to ride on oppositely disposed surfaces 63 of the rotor. Again, the skirts 65 of the carrier block straddle the rotor 61 and the lower edge surfaces 66 of the skirts abut the guide surfaces or sides 62 of the rotor block. The moment arms D and D again provide a ratio D /D which is greater than 1 thereby permitting less movement of the contact about the pivot points 63 on the rotor than movement D of the lower corner of the skirt 65 about the support tracks of the rotor.

While in accordance with the patent statutes, there has been described what at present are considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and its is, therefore, the aim of the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the invention.

What is claimed is:

1. A variable resistance device comprising:

a case;

a contact carrier having a support slidable on said support track of said rotor, said contact carrier having side skirts straddling opposite sides of said rotor and so disposed as to slide on said guide surfaces of said rotor, said skirts engaging said guide surfaces of said rotor at a distance D from the engagement point of said support track and said carrier;

a spring contact mounted on said carrier and engaging said helically wound coil, said spring contact urging said carrier into engagement with said support track of said rotor, the point of engagement of said spring contact and said helical coil being a distance D from the engagement point of said support track and said carrier, the distance D being at least equal to or greater than the distance D so that engagement of said side skirts with said guide surfaces of said rotor minimizes rocking movement 0 fsaid contact about said support track; and

means for advancing said carrier along said support track of said rotor thereby to move said spring contact into engagement with successive portions of said resistance coil as said rotor is rotated.

2. The variable resistance device defined in claim 1 in which:

said guide surfaces on said rotor are undercut on opposite sides of said shaft and form longitudinal shoulders; and said skirts of said carrier block extend around said shoulders formed on said rotor and abut against said guide surfaces and said undercut shoulders to retain said carrier in place on said rotor. 3. The variable resistance device defined in claim 1 in which:

said longitudinal support track of said rotor comprises a pair of beveled surfaces arranged in planes adapted to intersect at a first angle; and said support of said contact carrier comprises a pair of surfaces arranged in planes adapted to intersect at second angle, greater than said first angle. 4. The variable resistance device defined in claim 1 in which:

90 said rotor has a cross-sectional shape of an arrow with the point cut off, including opposed slanting surfaces forming said longitudinal support track, and undercut barb-shaped shoulders formed above said guide surfaces of said rotor; and

said contact carrier block is provided with a recess adapted to receive said arrow-shaped rotor with said skirts extending around opposite sides of said rotor and slidingly engaging said guide surfaces of said rotor directly adjacent said barb-shaped shoulders.

References Cited UNITED STATES PATENTS ROBERT K. SCHAEFER, Primary Examiner.

4 H. I. HOHAUSER, Assistant Examiner. 

